This invention relates generally to a femoral component of a hip joint prosthesis and, more particularly, is directed to the shape of the intramedullary stem which is inserted into the surgically prepared medullary canal of the proximal femur for fixation or anchorage of the prosthesis into the surrounding supportive cortical bone.
As is well-known in the art, the intramedullary stem or blade-like portion of the femoral component of a hip joint prosthesis is inserted into the prepared medullary canal of the proximal femur for fixation within the surrounding bone structure. This fixation occurs by either a bone cementing means which employs methyl methacrylate, or by a biological fixation means employing a mechanism of bone ingrowth within minute voids or porosities of a specially applied porous-like metal alloy surface structure This porous surface structure is metallurgically integrated onto the surface of the prosthesis stem at specially selected locations.
With this in mind, consistency or congruency of the shape of the femoral prosthesis stem, relative to the shape of the proximal medullary canal of the femur, is an important technical and clinical consideration for achieving a more uniform mantle thickness of interposed affixing bone cement which tends to maintain improved uniformity of the developed stresses under functional loading when a bone cement fixation means is employed and for achieving a tight mechanical press-fit of the intramedullary stem portion of the femoral component prosthesis within the adjacent supporting bone structure of the proximal medullary canal when a biological bone ingrowth fixation means is employed It is also known that a tight mechanical press-fit between the femoral prosthesis stem and adjacent bone provides the necessary prerequisites for achieving clinically reliable bone ingrowth fixation, namely, by providing close interface apposition of the adjacent bone with the porous structure of the prosthesis for considerations of enhanced bone ingrowth mechanics and by providing the mechanical stability to preclude significant micromotion at the bone/porous coating interface during the prolonged post-operative bone ingrowth period. The occurrence of interface micromotion is generally considered as a major deterrent for achieving bone ingrowth fixation and is a contributing factor in clinical failures associated with femoral prosthesis loosening.
Unfortunately, due to the wide variety of femurs, and more specifically, the shape of the proximal intramedullary canal from physiological size variations, as well as disease, trauma and congenital related factors, achieving a consistent tight and reliable pressfit of the prosthesis stem within the available adjacent bone structure of the proximal femur, without resorting to significant bone preparation sculpturing (if sufficient bone mass is available), is generally a difficult surgical accomplishment, requiring careful attention to surgical detail.
Many contemporary hip prosthesis femoral components, such as those described in U.S. Pat. Nos. 4,551,863; 4,840,632; 4,851,007; 4,908,035; 4,919,678; and 4,936,863 incorporate a straight, intramedullary stem design in the lateral view or anterior-posterior (A-P) plane. In this plane, the straight intramedullary stem of the femoral prosthesis component is seated and stably positioned within the curved proximal femur by selecting a stem of proper distal intramedullary stem diameter or distal size which is consistent with distal reaming to allow a three-point support by adjacent bone structure Attainment of a more complete stem fit in the lateral view or anterior-posterior plane of the proximal femur is technically difficult, due to the characteristic serpentine shape of the medullary canal in this plane. The shape of the proximal femur in the A-P view or medial-lateral plane is consistent with accepted means of canal preparation, that is, distally reaming the cortical bone, and also reaming the cancellous bone proximally-laterally, as well as rasping the cortical and cancellous bone proximal-medially. Additionally, the convergent shape of the medullary canal in the A-P view (M-L plane) not only facilitates surgical preparation of the medullary canal but also facilitates prosthesis stem insertion and achievement of a tight stem press-fit. The medullary stem of the femoral prosthesis should attain an exact fit with the endosteal surfaces of the proximal canal, as viewed in an A-P radiograph and the attainment of a three-point stem support in the lateral radiograph. The medial and lateral borders of the stem should intimately contact adjacent bone surfaces, along the entire medullary stem length, with minimal interface gaps, reflective of a tight and mechanically stable press-fit.
Of course, proper surgical technique and careful attention to detail in the preparation of all bony surfaces, relative to the geometric shape and dimensional size of the femoral prosthesis stem, must be carefully considered both preoperatively and at surgery to avoid inadvertent bone fracture upon attempted seating of the prosthesis.
Additionally, besides attempting to achieve a stable mechanical press-fit within the proximal femoral canal for the porous ingrowth fixation mode (and for the cemented mode, as well), the surgeon must attempt to position the center of the bearing head of the femoral prosthesis coincident with the articular center of the hip joint for duplication of the natural functional biomechanical characteristics of the hip. The two principal spacial components of femoral head position include vertical off-set distance (vertical elevation from the collar or neck resection level to the head center) for leg length considerations and horizontal offset (horizontal dimension between the medullary stem center-line and the head center) for attainment of proper hip joint function and power characteristics.
Some variability of femoral head position is generally provided to the surgeon by selection of a range of modular neck lengthening prosthesis head components which allow discrete incremental increases in both the vertical and horizontal off-sets. Variation in just a horizontal component of the femoral head position is achieved by selecting an alternative femoral component design, which incorporates a larger horizontal off-set. This may not be readily available for a given distal stem size within the same prosthesis system, and accordingly, may involve consideration of a long lead-time custom device, or more probably, consideration of another prosthesis system alternative may have to be explored. Variation in just the vertical femoral head off-set can be achieved by preparing the proximal femoral canal to accept the prosthesis at a higher position or by selecting an alternative femoral component design of proper distal diameter, but with a broader proximal-transverse or proximal medial-lateral (M-L) intramedullary stem width, allowing the prosthesis to seat at a higher position within the femur. The former technique for seating the femoral component at a higher position can result in larger stem/bone interface gaps proximal-medially, a condition reflective of a less desirable, and possibly less stable, stem fit. Conversely, to lower the vertical head off-set position, a femoral prosthesis stem of narrower proximal-transverse width can be employed, which allows the prosthesis to seat at a lower position within the femur. Lower prosthesis seating may also result in an inexact fit between the stem and bone, especially if the femur was surgically prepared for a larger proximal stem design of different proximal-medial curvature. Again, these adjustments in femoral head position must be surgically achieved while coincidentally attaining a tight, stable intramedullary stem fit within the proximal femur with minimal interface gaps between the stem and bone.
The attainment of a tight stem/bone fit is difficult to verify interoperatively, although some techniques of somewhat limited and questionable utility currently exists. For a good, reliable press-fit, there is no substitute for extensive surgical experience, exacting preoperative planning and design sensibility and consistency-between the shape of the intramedullary stem, the shape of the proximal medullary canal and the surgical instrumentation for preparing and shaping the femur bone bed to accept the prosthesis.
Still another prosthesis sizing consideration involves the variation of the proximal femoral transverse canal size relative to a given distal canal diameter and given femoral head position. Few contemporary prosthesis systems are designed to directly accommodate this commonly encountered dimensional canal variation. This situation is usually indirectly accommodated by inserting a proximally undersized or oversized prosthesis stem to achieve a so-called press-fit, and managing resultant lower or higher medullary stem position by selection of a modular neck length component for attainment of proper leg length adjustment, while accepting whatever horizontal femoral head off-set that results.
Still another stem sizing consideration involves the geometric curvature or shape of the proximal-medial intramedullary stem. It appears that many of currently available femoral prosthesis systems claim that the respective stem design is "physiologically" configured to achieve a consistent and mechanically stable "press-fit". However, all appear to reflect different geometric configurations and correspondingly, different levels of physiological conformity with the proximal femoral canal, when tested in a typical preoperative planning setting.